Michael Gove said the European Union is encouraging extremism across Europe as he joined five other cabinet ministers in breaking ranks with David Cameron to campaign to take Britain out of the EU.

The justice secretary, one of Cameron’s closest political allies, posed for photos with cabinet colleagues at the headquarters of Vote Leave just hours after the prime minister fired the starting gun for the UK’s four-month debate on EU membership.

Gove was joined by Iain Duncan Smith, the work and pensions secretary, John Whittingdale, the former Thatcher aide who is now culture secretary, Chris Grayling, the leader of the Commons, Theresa Villiers, the Northern Ireland secretary, and Priti Patel, the employment minister who attends cabinet.

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The “gang of six” posed next to a signed banner saying: “Let’s take back control” after travelling to the Vote Leave headquarters from a rare Saturday cabinet meeting in which Cameron made his case for the UK’s renegotiated membership of the EU. In a speech outside Downing Street after the meeting, the prime minister said the referendum would take place on 23 June.

The manoeuvre came after weeks of rumours about which Tory frontbenchers would break ranks.

A notable absentee from the rebellion, Boris Johnson, has kept Downing Street waiting about which way he will jump in the referendum campaign. Amid some irritation in No 10, the London mayor is expected to wait until the prime minister outlines his plans to reassert the sovereignty of parliament before announcing his intentions. Johnson said last week that he would endorse one side in the referendum campaign with a “deafening éclat” soon after the prime minister reached a deal in Brussels.

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The London mayor appears to have been wrongfooted by the prime minister’s decision to confirm within an hour of his deal in Brussels on Friday night that Gove would be campaigning for Brexit. The move by Gove puts immense pressure on Johnson to join the leave side. He had hoped that the prime minister’s new parliamentary sovereignty initiative would give him an option to campaign for remain.

Gove issued a 1,500-word statement in which he said that he has spent weeks wrestling with the most difficult decision of his political life. The former journalist wrote: “It pains me to have to disagree with the prime minister on any issue. My instinct is to support him through good times and bad.

“But I cannot duck the choice which the prime minister has given every one of us. In a few months’ time we will all have the opportunity to decide whether Britain should stay in the European Union or leave. I believe our country would be freer, fairer and better off outside the EU. And if, at this moment of decision, I didn’t say what I believe I would not be true to my convictions or my country.”

In his statement, which read like a personal political manifesto rather than the light-hearted interventions by Johnson, Gove said that Britain could follow the US in declaring its own independence. Gove said: “Instead of grumbling and complaining about the things we can’t change and growing resentful and bitter, we can shape an optimistic, forward-looking and genuinely internationalist alternative to the path the EU is going down. We can show leadership. Like the Americans who declared their independence and never looked back, we can become an exemplar of what an inclusive, open and innovative democracy can achieve.”

Chris Grayling on the phone to voters at the launch of the Vote Leave campaign. Photograph: WPA Pool/Getty Images

The justice secretary showed his unease for the EU by blaming it for the rise of extremism. “Far from providing security in an uncertain world, the EU’s policies have become a source of instability and insecurity. Razor wire once more criss-crosses the continent, historic tensions between nations such as Greece and Germany have resurfaced in ugly ways and the EU is proving incapable of dealing with the current crises in Libya and Syria … All of these factors, combined with popular anger at the lack of political accountability, has encouraged extremism, to the extent that far-right parties are stronger across the continent than at any time since the 1930s.”

Gove released his statement after what was described as a good-natured cabinet meeting lasting about two hours. All ministers – the 22 full members of the cabinet plus the eight ministers who attend every week – were allowed to speak in order of seniority.

Ministers on both sides of the divide agreed that they would need to campaign in a collegiate way. Duncan Smith, who made his name as a Maastricht rebel in the early 1990s, is understood to have made clear that there is no need nor any appetite for a repeat of the bitterness of those years.

Some ministers campaigning to leave the EU are even said to have acknowledged that the new deal negotiated by the prime minister was an improvement on the status quo.

Grayling described the cabinet as a “really constructive, grown-up, friendly” cabinet. But he suggested that the positive voices on his side about the prime minister’s EU deal were limited when he said that Cameron had simply won concessions in Brussels.

The leader of the Commons told the BBC: “I don’t in any way criticise him for what he’s done. He’s put in a herculean effort to try to deliver change, to get a deal. He has undoubtedly made some progress but what he has discovered over the last couple of days is the limitations of change that we can secure within the European Union.

“What the prime minister has done is bring back some concessions that will change parts of our relationship with the European Union. I’ve always supported the process of renegotiation. But what it doesn’t do is give us control of our borders.”